1. Field of the Invention
The present invention relates to a display device which consumes less electric power and which provides large screen brightness. In particular, the present invention relates to a display-driving device and a display-driving method for driving a display device for displaying a picture image corresponding to an image signal on an optical waveguide plate by controlling leakage light at a predetermined position of the optical waveguide plate by controlling the displacement action of an actuator element in a direction to make contact or separation with respect to the optical waveguide plate in accordance with an attribute of the image signal to be inputted.
2. Description of the Related Art
Those hitherto known as the display device include, for example, cathode ray tubes (CRT), liquid crystal display devices, and plasma displays.
Those known as the cathode ray tube include, for example, ordinary television receivers and monitor units for computers. Although the cathode ray tube has a bright screen, it consumes a large amount of electric power. Further, the cathode ray tube involves a problem that the depth of the entire display device is large as compared with the size of the screen. Further, for example, the cathode ray tube involves drawbacks in that the resolution is decreased in the circumferential areas of a display images, the image or the graphic is distorted, there is no memory function, and it is impossible to present display in a large scale.
The reason for the foregoing phenomenon is as follows. That is, in the case of the cathode ray tube, the electron beam emitted from the electron gun is greatly deflected. Therefore, the light emission point (beam spot) is expanded at portions at which the electron beam reaches the fluorescent screen of the Braun tube in an inclined manner, and thus the image is displayed in an inclined manner. For this reason, strain occurs on the display image. Moreover, there is a limit for the maintenance to keep a large space at the inside of a Braun tube to be in a vacuum.
On the other hand, the liquid crystal display device is advantageous in that the entire device can be miniaturized, and the display device consumes a small amount of electric power. However, the liquid crystal display device involves problems in that it is inferior in screen brightness, and the field angle of the screen is narrow. Further, since gradational expression is made in accordance with the voltage level, there is a drawback that the arrangement of the driving circuit becomes extremely complicated.
For example, when a digital data line is used, its driving circuit comprises a latch circuit for retaining component RGB data (each 8 bit) for a predetermined period, a voltage selector, a multiplexer for making change to provide voltage levels of the type corresponding to the number of gradations, and an output circuit for adding output data from the multiplexer to the digital data line. In this case, when the number of gradations becomes large, it is necessary for the multiplexer to perform the switching operation at an extremely large number of levels, in accordance with which the circuit arrangement becomes complicated.
When an analog data line is used, its driving circuit comprises a shift register for aligning, in the horizontal direction, component RGB data (each 8 bit) inputted successively, a latch circuit for holding parallel data from the shift register for a predetermined period, a level shifter for adjusting the voltage level, a D/A converter for converting output data from the level shifter into an analog signal, and an output circuit for adding the output signal from the D/A converter to the analog data line. In this case, an operational amplifier is used in the D/A converter. Thus, a predetermined voltage corresponding to the gradation is obtained. However, when the range of gradation becomes wide, it is necessary to use an operational amplifier which outputs a highly accurate voltage. Therefore, such a system involves a drawback that the structure becomes complicated, and the price also becomes high.
Since the plasma display has a small volume of its display section in the same manner as the liquid crystal display device. Therefore, the plasma display is advantageous in that it can be miniaturized, and it is easy to recognize the image because it has a flat display screen. Especially, the alternating current type plasma display additionally has an advantage that no refresh memory is required owing to the memory function of the cell.
By the way, in the case of the plasma display described above, in order to allow the cell to possess the memory function, it is necessary that the polarity of applied voltage is changed in an alternating manner so that the discharge is continued. For this reason, it is necessary for the driving circuit to comprise a first pulse generator for generating a sustain pulse in the X direction and a second pulse generator for generating a sustain pulse in the Y direction. For this reason, a problem arises in that the arrangement of the driving circuit is inevitably complicated.
The present invention has been made in consideration of such problems, an object of which is to provide a display-driving device and a display-driving method in which it is unnecessary to perform, for example, complicated voltage switch and voltage selection even when the range of display gradation is widened, it is possible to suppress the setting number of working voltages to the minimum, and it is possible to realize a simplified arrangement of a peripheral circuit system (including driving circuits).
Another object of the present invention is to provide a display-driving device and a display-driving method in which it is possible to exhibit the function as the display by maximally utilizing the memory function of a shape-retaining layer (piezoelectric/electrostrictive layer and anti-ferroelectric layer) of an actuator element for constructing a picture element (image pixel).
Still another object of the present invention is to provide a display-driving device and a display-driving method in which the selection period for a picture element is minimized so that the electric power consumption is effectively reduced.
Still another object of the present invention is to provide a display-driving device and a display-driving method in which the cross talk between picture elements during the unselection period is suppressed so that the stabilization of light emission and the stabilization of display brightness (gradation) are realized.
Still another object of the present invention is to provide a display-driving device and a display-driving method which is advantageous to extend the gradation level when the light-emitting rising time Tr of the picture element and the quenching falling time Tf of the picture element have a relationship of Tr greater than  greater than Tf.
According to the present invention, there is provided a display-driving device for driving a display comprising an optical waveguide plate for introducing light thereinto, and a driving section provided opposingly to one plate surface of the optical waveguide plate and including a number of actuator elements arranged corresponding to a large number of picture elements, for displaying, on the optical waveguide plate, a picture image corresponding to an image signal by controlling leakage light at a predetermined portion of the optical waveguide plate by controlling displacement action of each of the actuator elements in a direction to make contact or separation with respect to the optical waveguide plate in accordance with an attribute of the image signal to be inputted; the display-driving device comprising a first driving circuit for selecting the actuator elements at least in one row unit, a second driving circuit for outputting displaying information to the selected row, and a signal control circuit for controlling the first and second driving circuits; wherein the first and second driving circuits are controlled to perform gradation control in accordance with a temporal modulation system by using the signal control circuit; a light source turn on period and a light source turn off period are set within one field provided that a display period for one image is defined as the one field; an overall bending displacement period for making bending displacement of all of the actuator elements is set within the light source turn off period; and a gradational display period for performing substantial gradational display is set within the light source turn on period.
According to the present invention, the signal control circuit performs control such that the first driving circuit selects the actuator elements (picture elements) at least in one row unit, and the second driving circuit outputs the display information to the respective picture elements included in the selected row. At this time, the first and second driving circuits are controlled by the aid of the signal control circuit so that the display effected by the respective picture elements makes gradational expression at least in accordance with the temporal modulation system.
During this process, all of the actuator elements are subjected to the bending displacement in the overall bending displacement period within the light source turn off period in the one field. For example, if the light is introduced into the optical waveguide plate in this state, all of the picture elements may cause light emission. However, all of the picture elements are in the light off state, because the light source is turned off.
And then, the control is made for the respective picture elements to perform the substantial gradational display during the gradational display period in the next light source turn on period. The gradational display resides in gradational control based on the temporal modulation system. Therefore, it is unnecessary to perform complicated voltage switch and voltage selection even when the range of display gradation of the picture elements is widened. Thus, the setting number of working voltages can be suppressed to the minimum.
In general, the time required to allow the actuator element to make bending displacement until light emission is sometimes extremely longer than the time required to reset the bending displacement of the actuator element until quenching. In such a case, it is necessary to set a delay time until light emission within the gradational display period. Such a procedure causes a problem that it is disadvantageous to extend the gradation level.
However, in the display-driving device according to the present invention, all of the actuator elements are subjected to the bending displacement during the light source turn off period before the gradational display period is started. Therefore, the light emission is performed for a period of time corresponding to the gradation level of each of the picture elements in the next gradational display period. After that, the bending displacement of the actuator element corresponding to the concerning picture element is reset to successfully turn off the picture element. Accordingly, it is unnecessary to set any preparatory period (delay time) for making bending displacement of the actuator element during the gradational display period. This results in maximum utilization of the limited gradational display period, making it possible to obtain an effect of advantage to extend the gradation level of the picture element.
In the present invention, it is desirable that the first and second driving circuits have the following features.
(1) The actuator element undergoes the capacitive load. Therefore, considering the fact that the capacitive load is subjected to the driving, it is desirable that the partial voltage ratio, which is applied to the capacitive load, is not less than 50%, for example, at the time of completion of voltage (ON voltage) application for allowing the actuator element to make the bending displacement.
(2) In order to obtain an displacement amount of the actuator element which makes it possible to express the ON state and the OFF state of the picture element, it is desirable that an voltage output of not less than 20 V can be provided.
(3) It is desirable to consider the fact that the direction of the output current is recognized to be bidirectional.
(4) It is desirable that the load concerning the two-electrode structure in the row direction and the column direction can be subjected to the driving.
It is desirable for the display-driving device constructed as described above that the actuator element of the display comprises a shape-retaining layer, an operating section having at least a pair of electrodes formed in contact with the shape-retaining layer, a vibrating section for supporting the operating section, and a fixed section for supporting the vibrating section in a vibrating manner; wherein the display comprises a displacement-transmitting section for transmitting the displacement action of the actuator element to the optical waveguide plate, the displacement action being generated by voltage application to the pair of electrodes. In the present invention, the term xe2x80x9cactuator element having the shape-retaining layerxe2x80x9d indicates an actuator element which has at least two or more displacement states at an identical voltage level.
Accordingly, all of the light, which is introduced, for example, from the end of the optical waveguide plate, is totally reflected at the inside of the optical waveguide plate without being transmitted through the front and back surfaces of the optical waveguide plate (OFF state), by regulating the magnitude of the refractive index of the optical waveguide plate. In this state, when the displacement-transmitting section contacts with the back surface of the optical waveguide plate at a distance of not more than the wavelength of the light, then the light, which has been totally reflected, is transmitted to the surface of the displacement-transmitting section contacting with the back surface of the optical waveguide plate. The light, which has once reached the surface of the displacement-transmitting section, is reflected by the surface of the displacement-transmitting section, and the light behaves as scattered light. A part of the scattered light is reflected again at the inside of the optical waveguide plate. However, almost all of the scattered light is not reflected by the optical waveguide plate, and the light is transmitted through the front surface of the optical waveguide plate (ON state).
As described above, it is possible to control the presence or absence of light emission (leakage light) at the front surface of the optical waveguide plate, depending on the presence or absence of the contact of the displacement-transmitting section disposed at the back of the optical waveguide plate. In this case, one unit for allowing the displacement-transmitting section to make the displacement action in the direction to give contact or separation with respect to the optical waveguide plate may be regarded as one picture element. Thus, a picture image (for example, characters and graphics) corresponding to an image signal can be displayed on the front surface of the optical waveguide plate in the same manner as the cathode ray tube and the liquid crystal display device, by arranging a large number of such picture elements in a matrix form, and controlling the displacement action of each of the picture elements in accordance with an attribute of the inputted image signal.
The actuator element having the shape-retaining layer has the following features.
(1) The threshold characteristic concerning the change from the OFF state to the ON state is steep as compared with the case in which no shape-retaining layer exists. Accordingly, it is possible to narrow the deflection width of the voltage, and it is possible to mitigate the load on the circuit.
(2) The difference between the ON state and the OFF state is distinct, resulting in improvement in contrast.
(3) The dispersion of threshold value is decreased, and an enough margin is provided for the voltage setting range.
It is desirable to use, as the actuator element, an actuator element which makes, for example, upward displacement (giving the separated state upon no voltage load and giving the contact state upon voltage application) because of easiness of control. Especially, it is desirable to use an actuator element having a structure including a pair of electrodes on its surface. It is preferable to use, for example, a piezoelectric/electrostrictive layer and an anti-ferroelectric layer as the shape-retaining layer.
It is also preferable for the display-driving device constructed as described above that the gradational display period comprises a plurality of subfields, a selection period and an unselection period are set for each of the subfields, and any of operations of maintenance of light emission/quenching is performed in accordance with a gradation level of the picture element upon selection of the picture element.
In this embodiment, the operation of maintenance of light emission is performed in each selection period ranging from the first subfield to the subfield of a number corresponding to the gradation level of the picture element, of the plurality of subfields, and the operation of quenching is performed in each selection period for the following subfields.
It is also preferable for the display-driving device constructed as described above that the first driving circuit is subjected to timing control by the signal control circuit so that all row selection is completed within each of the subfields by using the first driving circuit, and the second driving circuit is subjected to timing control by the signal control circuit so that a data signal, which is prepared by allotting a display time corresponding to each gradation level to an effective display period of each subfield, is outputted during the selection period of each subfield, for each of the picture elements concerning the selected row, by using the second driving circuit.
According to this embodiment, at first, the group of picture elements disposed in the first row are selected by the first driving circuit upon the start of one field. The data signal is supplied to the group of picture elements in the first row by the aid of the second driving circuit. The data signal supplied to each of the picture elements is a data signal (for example, ON signal and OFF signal) prepared by allotting the display time corresponding to the gradation level to each of the subfields. When one picture element is observed, the display time corresponding to the gradation level of the picture element is assigned to the time width allotted to each of the subfields. This procedure includes a case in which the display time is assigned to all of the subfields, and a case in which the display time is assigned to some of the subfields.
Specifically, for example, when one field is divided into four subfields (first to fourth subfields), the following assignment is available. That is, the time width of the continuous first to fourth subfields is 4, the time width of the continuous first to third subfields is 3, the time width of the continuous first and second subfields is 2, and the time width of the first subfield is 1.
Therefore, for example, the gradation level of the picture element is 4, all of the subfields are selected. When the gradation level is 2, the first and second subfields are selected.
Those adoptable as the output form of the data signal supplied to the picture element include, for example, a form in which the ON signal is outputted to the selected subfield, and the OFF signal is outputted to the unselected subfield.
It is desirable that each of the first and second driving circuits comprises only one driving circuit, i.e., the first and second driving circuits comprise only two driving circuits. The use of the only two driving circuits is sufficient, because the actuator element has the structure composed of the two electrodes (pair of electrodes), and it has the shape-retaining function.
It is also preferable that a voltage sufficient to maintain the bending displacement of the actuator element is applied to the actuator element corresponding to an objective picture element within the selection period during the operation of maintenance of light emission, and a voltage sufficient to reset the displacement of the actuator element is applied to the actuator element corresponding to the objective picture element within the selection period during the operation of quenching.
In another embodiment, it is also preferable that a direction for scanning the picture element in each of the subfields is different between the fields adjacent to one another. In this embodiment, it is possible to avoid occurrence of discrepancy corresponding to one gradation between the picture element in the first row and the picture element in the final row, making it possible to improve the image quality.
It is preferable for the display-driving device constructed as described above that at least a reset period for making display brightness to be substantially zero is provided between a certain gradational display period and the next gradational display period. Accordingly, the display brightness is once made zero during the reset period. Therefore, it is easy to respond to the display of an animation image.
It is desirable for the display-driving device constructed as described above that the first driving circuit is capable of setting at least three voltage levels, and the second driving circuit is capable of setting at least two voltage levels.
In another preferred embodiment, the selection of the row is performed by the first driving circuit by outputting a selection pulse signal during the selection period, and outputting an unselection signal during the unselection period; and the output of the data signal is performed by the second driving circuit by outputting an ON signal during the selection period of an allotted subfield of the respective subfields, and outputting an OFF signal during the selection periods of the other subfields.
In this embodiment, a voltage sufficient to maintain the bending displacement of the actuator element is applied to the actuator element corresponding to an objective picture element within the output period of the ON signal, and a voltage sufficient to reset the displacement of the actuator element is applied to the actuator element corresponding to the objective picture element within the output period of the OFF signal.
Accordingly, the sufficient voltage to maintain the bending displacement of the actuator element is applied to the picture element during the selection period for the subfield selected by assigning the time width of the gradation level for one picture element. Therefore, the bending displacement is maintained for the concerning actuator element owing to the voltage application. Thus, the occurrence of leakage light (light emission) from the optical waveguide plate as described above is maintained. The bending displacement state is stored until the voltage is applied in the opposite direction (until the OFF signal is supplied).
The unselection signal is outputted during the unselection period after the selection period. In this case, the unselection signal may be a signal fixed at a voltage smaller than the voltage used during the selection period, or the unselection signal may be a signal which fluctuates in an alternating manner. Accordingly, the state of the bending displacement in one direction is maintained for the actuator element during the unselection period.
As for the unselected subfield, the voltage sufficient to reset the bending displacement of the concerning actuator element is applied to the concerning picture element during the selection period. Accordingly, the concerning picture element is in the state of the lowest brightness (quenching).
It is also preferable that the first driving circuit outputs a selecting window pulse for applying the voltage sufficient to maintain the bending displacement of the actuator element to the actuator element of an objective picture element by means of combination with the ON signal during the selection period. Alternatively, it is also preferable that the first driving circuit outputs a signal for applying the voltage sufficient to reset the bending displacement of the actuator element to the actuator element of an objective picture element by means of combination with the OFF signal during the selection period.
It is desirable for the display-driving device constructed as described above that phase information is added at least to the OFF signal so that a difference in average voltage applied during the unselection period to the actuator element of each of the picture elements is decreased.
Ideally, during the unselection period, it is desirable to apply a fixed electric potential in a degree not to affect the bending displacement as described above, because it is necessary to maintain the bending displacement state of the actuator element as it is.
However, since all of the row selection is completed in each subfield, the data signal (ON signal and OFF signal) for another row successively appears during the unselection period of each subfield. That is, when observation is made for one picture element, the voltage waveform of the concerning picture element during the unselection period is determined by the pattern of the data signal (appearance pattern of the ON signal and the OFF signal) for the row other than the row to which the concerning picture element belongs, in the column to which the concerning picture element belongs.
For example, when the ON signal is outputted for all of the rows other the row including the concerning picture element, the average voltage of the concerning picture element during the unselection period is fixed at a voltage level (conveniently referred to as xe2x80x9chigh voltage levelxe2x80x9d) obtained by subtracting the reference level from the voltage level of the ON signal. When the OFF signal is outputted for all of the rows other the row including the concerning picture element, the average voltage is fixed at a voltage level (conveniently referred to as xe2x80x9clow voltage levelxe2x80x9d) obtained by subtracting the reference level from the voltage level of the OFF signal. When the ON signal and the OFF signal are alternately outputted in the unit of row for all of the rows other the row including the concerning picture element, the average voltage is at an intermediate voltage between the high voltage level and the low voltage level.
As a result, the bending displacement of the actuator element during the unselection period is delicately changed depending on the voltage change (voltage change depending on the pattern of the ON signal and the OFF signal). Especially, when the ON signal or the OFF signal is collectively outputted for a large number of rows, the difference in average voltage is large. Therefore, there is a possibility that the display state (brightness and gradation) may become unstable during the unselection period for the concerning picture element.
When the ON signal and the OFF signal appear in an alternating manner, then the waveform of the average voltage in this case is not fixed at the intermediate voltage as described above, and it fluctuates while giving a certain offset, because the pulse width of the ON signal and the OFF signal is approximately the same as the selection period.
Thus, in the present invention, in order to solve the problem described above, the phase information is added to the selection pulse signal and the ON signal and/or the OFF signal respectively.
Accordingly, both of the ON signal and the OFF signal constitute a pulse signal including the high level and the low level which exist in a mixed manner during the period corresponding to the selection period.
Therefore, as described above, when consideration is made for one picture element, the pulse signal having a narrow pulse width, in which the amplitude is (high voltage levelxe2x80x94low voltage level), continuously appears during the unselection period for the concerning picture element in all cases including, for example, a case in which the ON signal is outputted for all of the rows, a case in which the OFF signal is outputted for all of the rows, and a case in which the ON signal and the OFF signal are alternately outputted in the unit of row. As a result, the average voltage during the unselection period does not depend on the pattern of the ON signal and the OFF signal, and it has an approximately constant value. Therefore, the display state (brightness and gradation) during the unselection period is stabilized.
According to another aspect of the present invention, there is provided a display-driving method for driving a display comprising an optical waveguide plate for introducing light thereinto, and a driving section provided opposingly to one plate surface of the optical waveguide plate and including a number of actuator elements arranged corresponding to a large number of picture elements, for displaying, on the optical waveguide plate, a picture image corresponding to an image signal by controlling leakage light at a predetermined portion of the optical waveguide plate by controlling displacement action of each of the actuator elements in a direction to make contact or separation with respect to the optical waveguide plate in accordance with an attribute of the image signal to be inputted; the display-driving method comprising the steps of selecting the actuator elements at least in one row unit; outputting displaying information to the selected row; and making gradation control for each of the picture elements in accordance with a temporal modulation system; wherein a light source turn on period and a light source turn off period are set within one field provided that a display period for one image is defined as the one field; an overall bending displacement period for making bending displacement of all of the actuator elements is set within the light source turn off period; and a gradational display period for performing substantial gradational display is set within the light source turn on period.
Accordingly, it is unnecessary to perform, for example, complicated voltage switch and voltage selection even when the range of display gradation is widened, it is possible to suppress the setting number of working voltages to the minimum, and it is possible to realize a simplified arrangement of a peripheral circuit system (including driving circuits).
Even when the time required to allow the actuator element to make bending displacement until light emission is extremely longer than the time required to reset the bending displacement of the actuator element until quenching, it is possible to maximally utilize the limited gradational display period, providing an effect of advantage to extend the gradation level of the picture element.
In the method described above, the gradational display period comprises a plurality of subfields, a selection period and an unselection period are set for each of the subfields, and any of operations of maintenance of light emission/quenching is performed in accordance with a gradation level of the picture element upon selection of the picture element.
In this embodiment, timing control is performed by the signal control circuit so that all row selection is completed within each of the subfields, and a data signal, which is prepared by allotting a display time corresponding to each gradation level to an effective display period of each subfield, is outputted during the selection period of each subfield, for each of the picture elements concerning the selected row.
It is preferable that a voltage sufficient to maintain the bending displacement of the actuator element is applied to the actuator element corresponding to an objective picture element within the selection period during the operation of maintenance of light emission, and a voltage sufficient to reset the displacement of the actuator element is applied to the actuator element corresponding to the objective picture element within the selection period during the operation of quenching.
Especially, in order to avoid occurrence of discrepancy corresponding to one gradation between the picture element in the first row and the picture element in the final row, it is preferable that a direction for scanning the picture element in each of the subfields is different between the fields adjacent to one another.
It is preferable that at least a reset period for making display brightness to be substantially zero is provided between a certain gradational display period and the next gradational display period. By doing so, the display brightness is once made zero during the reset period. Therefore, it is easy to respond to the display of an animation image.
It is desirable for the method described above that at least three voltage levels are capable of being set upon the selection of the row, and at least two voltage levels are capable of being set upon the output of the displaying information.
In another preferred embodiment, upon the selection of the row, a selection pulse signal is outputted during the selection period, and an unselection signal is outputted during the unselection period; and upon the output of the displaying information, an ON signal is outputted during the selection period of an allotted subfield of the respective subfields, and an OFF signal is outputted during the selection periods of the other subflelds.
It is preferable for the method described above that a voltage sufficient to maintain the bending displacement of the actuator element is applied to the actuator element corresponding to an objective picture element within the output period of the ON signal, and a voltage sufficient to reset the displacement of the actuator element is applied to the actuator element corresponding to the objective picture element within the output period of the OFF signal.
It is also preferable that a selecting window pulse for applying the voltage sufficient to maintain the bending displacement of the actuator element is outputted to the actuator element of an objective picture element by means of combination with the ON signal during the selection period. Alternatively, it is also preferable that a signal for applying the voltage sufficient to reset the bending displacement of the actuator element is outputted to the actuator element of an objective picture element by means of combination with the OFF signal during the selection period.
It is preferable that phase information is added at least to the OFF signal so that a difference in average voltage applied during the unselection period to the actuator element of each of the picture elements is decreased. Specifically, it is preferable that the phase information is added to the selection pulse signal and the ON signal and/or the OFF signal respectively.
As explained above, according to the display-driving device and the display-driving method concerning the present invention, it is unnecessary to perform, for example, complicated voltage switch and voltage selection even when the range of display gradation is widened, it is possible to suppress the setting number of working voltages to the minimum, and it is possible to realize a simplified arrangement of a peripheral circuit system (including driving circuits).
Further, it is possible to exhibit the function as the display by maximally utilizing the memory function of the shape-retaining layer (piezoelectric/electrostrictive layer and anti-ferroelectric layer) of the actuator element for constructing the picture element (image pixel).
Furthermore, the selection period for the picture element is minimized so that the electric power consumption is effectively reduced, and the cross talk between the picture elements during the unselection period is suppressed so that the stabilization of light emission and the stabilization of display brightness (gradation) may be realized.
The present invention is advantageous to extend the gradation level when the light-emitting rising time Tr of the picture element and the quenching falling time Tf of the picture element have a relationship of Tr greater than  greater than Tf.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.